The present invention relates to a semiconductor laser device used for optical communications and optical information processing, etc., in which analog or digital signals are transmitted via optical fiber cables. When transmitted a long distance, an optical signal will decay. For this reason it is desirable to amplify the signal on the way to the destined place.
An optical fiber amplifier was developed for that purpose, and may be able to amplify an optical signal by 1,000 times at a wavelength of 1.55 .mu.m. The amplifier comprises an optical fiber of ca. 50 m in length, doped with erbium (hereafter referred to as erbium-doped optical fiber), and a semiconductor laser device with an optical output power of more than 150 mW at a wavelength of 1.48 .mu.m or 0.98 .mu.m (hereafter referred to as an excitation-use semiconductor laser device).
When a light from the excitation-use semiconductor laser device is applied to the erbium-doped optical fiber, electrons within erbium atoms are excited, and accumulate in excited states. An incidence of an optical signal upon the erbium-doped optical fiber gives rise to an stimulated emission of light, thereby resulting in an amplification of the optical signal. Therefore, the stronger the optical output of an excitation-use semiconductor laser device is, the more electrons are excited, thereby resulting in an enhanced amplification.
FIG. 1 shows a sectional view of a conventional excitation-use semiconductor laser device, and is described, for example, in Advanced Program of Optical Fiber Communications Conference 1992, page 45 which is incorporated herein by reference. In FIG. 1, 101 is an InGaAsP multiple-quantum-well (MQW) layer emitting a ray of laser light of 1.48 .mu.m in wavelength. The MQW layer comprises three layers: The intermediate layer is an active layer emitting light, consisting of five strained InGaAsP well layers of 2.4 to 6.8 nm in thickness and InGaAsP barrier layers lying therebetween and of 1.2 .mu.m in composition wavelength, where a composition wavelength is referred to as a wavelength of a light with an energy of the forbidden band of the relevant crystal. The remaining two layers are waveguide layers of 150 nm in thickness and 1.2 .mu.m in composition wavelength. 102, 103, 104, and 105 are p-type InP clad layer, n-type InP clad layer, n-type current-blocking layer, and p-type InP current-blocking layer, respectively.
The refraction indexes of these layers are lower than that of the multiple-quantum well layer 101. The n-type InP clad layer 103 and the multiple-quantum-well layer 101 form a concave shape as shown in the figure. Hereafter, this shape is referred to as a mesa. The mesa is extended vertically to the plane of the paper in the shape of a stripe. Hereafter, the shape is referred to as a stripe.
The horizontal distribution of refractive index which laser light senses is shown in the lower figure of FIG. 2(b), where O in the horizontal axis represents the center of the active layer. The refractive index is large at the mesa. On the other hand, as shown in FIG. 2(a), the optical intensity distribution of laser light has a fundamental mode of single peak type being maximum at the center of the active layer. In this way, laser light travels along the MQW layer 101.
The current-blocking layers 104 and 105 are disposed in such a way so that they serve as a reverse biased p-n junction to prevent a leakage current 115, thereby evading current flow anywhere other than the MQW layer 101. Another leakage current 110 is observed which passes through a p-type InP current-blocking layer 105.
103 is a p-type InGaAs contact layer, and enables reducing contact resistance with a p-type electrode 107. 108 is an n-type InP substrate and 109 is an n-type electrode.
The light-emitting side of the device is coated by a low refractive film with a refraction index of 3%, while the opposite side is coated by a high refractive film with a refractive index of more than 80%, thereby enabling large yield of laser light at the light-emitting side. The length of a resonator is 900 .mu.m.
FIG. 3 shows the characteristics of optical output versus current at a temperature of 25 degree centigrade, wherein an optical output of 250 mW is available at a current of 1 A.
As already mentioned, in order to enhance the amplification ratio of an optical fiber, it is necessary to increase the output of laser light of an excitation-use semiconductor laser device. However, the output of laser light goes to saturation due to its own thermal generation. Thus, it is desirable to make a structure that effectively liberates heat.
In order to make such a structure, the length of a resonator is extended so as to increase a contact area between the device and a heat sink. In the above mentioned example, an optical output of 310 mW is available with a resonator length of 1.5 mm.
However, an extended resonator results in a drop in outward differential quantum efficiency. Therefore, when the resonator is too long, an injection current is typically increased, so as to obtain a desired optical output, thus leading to an increase in heat generation, and eventually to a drop in optical output.
An alternative solution might be to increase the width of the MQW layer. This approach is based on the two reasons: one is that due to the broad path of a current, heat generation is constrained; the other is that due to the broad path of heat, efficiency of heat liberation is improved.
However, if the MQW layer 101 of FIG. 1 is extended, a difference in refractive index is too big, as shown in FIG. 4(b), and the transverse mode will shift to be multiple, that is, the optical intensity distribution exhibits a fundamental mode of double peak type. In this condition, the emitting light is split into two beams, thus making difficult a connection to an optical fiber.
The other problem is the reduction of the above described two kinds of leakage currents. Although the leakage currents are a small part of the total current, the total current amounts to as large as 1 ampere. Thus thermal generation by the leakage currents cannot be neglected. Furthermore, this temperature increase causes a drop in the standup, or forward voltage of the junction formed between the p-type InP current-blocking layer 105, and the n-type InP clad layer 103 and the n-type InP substrate 108, thus accelerating the increase of the leakage current 110.
Still furthermore, the increase of the leakage current 110 results in a gradual current increase through a thyristor structure formed by the p-type InP clad layer 102, the n-type InP current-blocking layer 105, and the p-type InP current-blocking layer 105, and the n-type InP clad layer 103 and the n-type InP substrate 108, thus further enhancing thermal generation.